I think——（this,that,it）is very useful to learn english well.填那个,

I think——（this,that,it）is very useful to learn english well.填那个,
I think——（this,that,it）is very useful to learn english well.填那个,

I think——（this,that,it）is very useful to learn english well.填那个,
绝对it啦~给分给分~

it 形式宾语

it

it，I是主语，think是谓语，后边是it引导的从句

选it！